A conventional laser beam level instrument as disclosed in U.S. Pat. No. 4,993,161 includes a laser diode mounted on a pendulous platform suspended from a frame which moves freely under the influence of gravity to provide automatic self-levelling. A vertical laser beam emitted from the diode is deflected by a prism which is rotated as driven by a motor, thereby providing a levelling reference through a horizontal plane.
However, such an instrument requires a rotating mechanism such as driven by a motor and transmission belt which may cause the drawback of heavy weight, increased production cost and maintenance problems.
The present inventor has invented an optical levelling and plumbing instrument without the rotating mechanism.